In motor vehicles, during the traveling along a curve, the turning radiuses of front wheels become larger than those of rear wheels, so that the traveling distance of the front wheels become longer than those of the rear wheels. For this reason, in a four-wheel drive vehicle of the type that front and rear wheels are drivingly connected to be driven by an engine, a problem arises in that since the difference between the traveling distances of the front and rear wheels becomes large during the traveling along a curve with a small radius, a tight-corner braking phenomenon to act as if braking has been applied to the front wheels occurs to exert an adverse influence on the steerability. As measures to obviate the problem, there is known a technology which is disclosed in Japanese unexamined, published patent application No. 8-2278. This technology is to prevent the occurrence of the tight-corner braking phenomenon in the following way. That is, the wheel speeds of four wheels are detected by wheel speed sensors, and then, the speed difference is calculated between the outside front wheel and the inside rear wheel. When the speed difference is equal to or larger than a judgment value which corresponds to a speed difference made on a curve with a turning radius causing the tight-corner braking to occur and when the speed difference is smaller than another judgment value which corresponds to another speed difference made on a curve with the minimum turning radius, the motor vehicle is judged to be in a turning state with a large steering angle, so that the transmission torque of a limited differential clutch (serving as differential control device) for coupling the front and rear wheels is decreased to prevent the occurrence of the tight-corner phenomenon.
However, the foregoing technology involves the following problems. That is, as shown in FIG. 7, when the motor vehicle M which was traveling along a curve with a turning radius equal to or smaller than a turning radius (hereafter simply as “tight-corner turning radius) which causes the tight-corner braking phenomenon to occur, is stopped temporarily at a position (A) and then is restarted, the tight-corner braking phenomenon occurs to adversely influence the steerability until the motor vehicle reaches another position (B) advanced somewhat from the stop position (A). Further, the motor vehicle suffers a shock when the tight-corner braking phenomenon is released at the position (B).
The causes for this are explained as follows. The rotational speeds of the wheels detected by the wheel speed sensors decrease to zero when the motor vehicle is stopped temporarily at the position (A), and since the judgment that the vehicle is in a turning state along a curve with a radius smaller than the tight-corner turning radius is reversed, the transmission torque of the differential control device is increased to bring the front and rear wheels into direct connection. Accordingly, by detecting the rotational speeds of the wheels by the wheel speed sensors after the restarting and then by calculating the speed difference between the outside front wheel and the inside rear wheel, it is judged that the vehicle is turning with a large steering angle to travel along a curve of a smaller radius than the tight-corner turning radius, and until so judged, the tight-corner braking phenomenon occurs to accumulate its stress in a power transmission mechanism between the front and rear wheels due to the competing rotations of the front and rear wheels depending on the difference between the traveling distances. Then, since it is judged at the position (B) that the vehicle is turning with a large steering angle along a curve of a smaller radius than the tight-corner turning radius, the transmission torque of the differential control device is decreased to release the tight-corner baking phenomenon. At this time, the stress accumulated in the power transmission mechanism is released instantaneously, so that the shock is given. This problem occurs not only when the motor vehicle M is restarted after being stopped temporarily, but also when the vehicle speed is increased again after being lowered to decrease the wheel speeds of the respective wheels to lower speeds than those which cannot be detected by the wheel speed sensors.
The object of the present invention is to solve the aforementioned various problems by performing a control of preventing the tight-corner braking phenomenon from occurring from the beginning of a restarting when the vehicle is restarted after being stopped temporarily in a control state that the occurrence of the tight-corner braking phenomenon is being prevented by decreasing the transmission torque of a differential control device for coupling front and rear wheels.